Messier 31
Astro-Physics 5-inch F6 @ F4.7
Losmandy G11 unguided
51, 1-minute exposures.
Canon 50D, ISO 1250, RAW
No darks, single twilight flat.

Auto-aligned and stacked using entropy-weighted averaging in DeepSky Stacker v3.2.2
Flat applied (gaussian blurred, inverted, multiplied), histogram-stretched, and down-sampled for display in Photoshop CS4.
Worth noting: the JPEG above is 1/160th of the filesize of the finished image.

Focussed using dual triangular aperture masks (thanks, Ron Wodaski).
Typical FWHM point spread: <4 pixels at 1.7 arcseconds per pixel.
Note that the 50D has 4.7 micron pixels and the Airy disk of this imaging system (at 550nm) is 6.3 microns, so there is room for improvement in achieved PSF.
A slight declination drift (2-3 arcseconds) was corrected in Photoshop.

The limiting magnitude is around 18.5 based on examining M31's globulars found in this image using Bill Gray's Guide 8 software. Assuming a distance modulus of -24.5 for M31, I should be able to detect novae in M31 and also its brightest stars: any star in M31 with an absolute magnitude of -6 or brighter, clear of intervening dust, should be visible. M31's counterparts to Rigel (-8.1), Betelgeuse and Deneb (-7.2), for example should be detected as individual stars. The granularity in NGC 206, the starcloud at upper right, is due in part to its blue supergiants (which are reported to be around 17th magnitude). Several hundred of M31's stars surely appear in the nearest spiral arm, but any one of the stellar images within that arm might be a faint foreground star.

The problem with unambiguously imaging individual stars in M31 isn't detection but identification. Among M31's stars is the brilliant S Doradus class variable star AF Andromedae. It shines at about -9 at maximum and -7 at minimum. Using Guide 8 and an auto-generated G-scale AAVSO chart, it is readily identified in the full-resolution image (but only marginally in the downsampled JPEG above). That makes AF AND literally one in a trillion.

Note two faint satellite trails in this photo.
Click here for a brighter satellite trail and a close up of the ISS using the same equipment.

December 2, 2009, Connellys Springs, North Carolina